1. Field of the Invention
The present invention relates to imaging tomography apparatuses, in particular for medical examinations, in particular in the fields of x-ray computed tomography (CT), single photon emission tomography (SPECT) and positron emission tomography (PET).
The invention also concerns a method for operation of a tomography apparatus of the type having a scanning unit that can rotate around a system axis and a bearing device for an examination subject.
The invention also concerns a tomography apparatus with a scanning unit that can be rotated around a system axis, a control device for activation of the scanning unit and a bearing device for an examination subject.
2. Description of the Prior Art
The conventional implementation of examinations of a number of examination subjects or patients by means of an x-ray computed tomography apparatus is as follows:                1. generation of an x-ray shadow image (topogram, scanogram, scout view) with a non-rotating x-ray source, with a first examination subject moved on the bearing device relative to x-ray source and detector system in the direction of the system axis,        2. definition of the region (scan region) of the first examination subject to be acquired in the direction of the system axis in the actual examination on the basis of the generated x-ray shadow image,        3. positioning of the first examination subject by moving the bearing device to the start point of the scan region,        4. start of the rotation of the x-ray source around the system axis,        5. implementation of the actual examination of the first examination subject in the form of a slice and/or volume scan with a rotating x-ray source, for the most part in the form of a spiral scan,        6. interruption of the rotation of the x-ray source and        7. repetition of the steps 1-6 with the next examination subject.        
Among other things, the topogram should allow minimization of the radiation dose by avoiding unnecessary, projection data from being acquired (for example in the spiral scan). Thus the topogram covers a larger region than would then be actually necessary for the subsequent image reconstruction in the desired region. Moreover, the topogram serves as the documentation of the scanned region.
This conventional method has disadvantages:                a) The entire workflow proves to be relatively long, which is undesirable for efficiency reasons and for medical reasons, in particular when the subject is an emergency patient.        b) If x-ray shadow images are desired from various projection directions (viewing angles), for example “from the front” and “from the side”, the patient is moved three times with the bed, namely twice for generation of the shadow images and a third time for the spiral scan.        
For optimization of the workflow and the flexibility, so-called “growing topograms” have been proposed, whereby an x-ray shadow image is acquired and displayed simultaneously or inline with the spiral scan, the x-ray shadow image growing with the spiral scan. Such a procedure is described in DE 198 02 405 A1 for a computed tomography apparatus with a 2-tube system. Computed tomography apparatuses with only one x-ray radiator have also been proposed in which the topogram likewise exists quasi-“online” by extraction of data that accrue during the actual scanning from a number of directions, for example in the spiral scan. Such methods are known from EP 0 531 993 B1, DE 41 03 588 C1 and DE 199 25 395 A1.
From the cited documents it is also known to allow one or two x-ray tubes to emit radiation only in a pulsed fashion at the projection angle or angles necessary for a tomogram.
Moreover, in EP 1 116 475 A1 a synthetic topogram is proposed that is generated by, initially, a 3D data set being reconstructed from the projection data sets and the shadow image being subsequently calculated from the 3D data set.